This relates generally to imaging systems, and more particularly to imaging systems with bond pad structures.
Modern electronic devices such as cellular telephones, cameras, and computers often use digital image sensors. Imaging systems (i.e., image sensors) often include a two-dimensional array of image sensing pixels. Each pixel typically includes a photosensitive element such as a photodiode that receives incident photons (light) and converts the photons into electrical signals. The imaging system contains an image sensor die with an image sensor integrated circuit and an array of photodiodes formed in a semiconductor substrate. Alternatively the imaging system contains an image sensor die having an array of photodiodes formed in a semiconductor substrate and the image sensor die is mounted on a digital signal processor (DSP) die. In both cases the face of the image sensor die is attached to a carrier substrate. In the first case the substrate is a piece of silicon without any circuitry and in the second case the substrate has the circuitry of the DSP. This second case is also known as a stacked sensor. The back side of the image sensor die is then processed through multiple steps including thinning, wet etch, CMP, passivation, light shield, bond pad formation, color-filter-array (CFA) and micro-lens. This type of image sensor is also known as a backside illumination (BSI) image sensor.
A metal light shield is formed on the backside of the BSI sensor to block light to the reference pixels (i.e. the optically black pixels). The metal light shield also blocks stray light from entering the periphery of the die. This same metal light shield can be extended into the pixel area in the form of a grid. This metal light grid reduces the cross-talk between the pixels, and blocks unwanted high angle light from entering the pixels. If the high angle stray light is not mitigated, it can cause “purple flare”.
A metal bond pad needs to be formed on the back side of the BSI sensor to connect the circuitry in the BSI sensor to the circuit board or package. This metal bond pad allows the usage of wirebonds or gold stud bumps to connect to the circuit board or the sensor package.
The process of creating the bond pad metal is generally separate from the process of creating the light shield metal or the light grid metal. It is desirable to reduce the number of process steps to reduce the cost of production. The standard methods also create high topography in the form of the bond pad metal thickness that projects up from the wafer surface. This topography can increase resist streaking during subsequent CFA and ulens processing which leads to yield loss.
It would therefore be desirable to provide improved ways of manufacturing image sensor dies.